feat(dashboard): auf jedem Bildschirm dasselbe Bild – Leinwand je Reiter, massstaeblich skaliert

Je Reiter wird die Flaeche des ersten Desktop-Bildschirms als __canvas im
Layout-JSON gespeichert (ohne API/DB-Aenderung, GRID_VERSION bleibt 3).
Das Raster rendert in Leinwandbreite und wird per transform: scale(min(bw/cw, bh/ch))
eingepasst, Schrift eingeschlossen, ohne Scrollen; unter 768 px wie bisher.
Ziehen/Groesse aendern unter Skalierung ueber eine eigene Positionsstrategie
(createScaledStrategy aus react-grid-layout 2.2.3 rechnet den Rasterversatz falsch).
Im Browser nachgewiesen: 1920x1080 -> 1366x768 (Faktor 0,66), Ziehen +200 px
folgt der Maus, 700 px ohne Skalierung.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/web/src/lib/grid-layout-migration.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import {
+  GRID_CANVAS_KEY,
   GRID_SCALE_FACTOR,
   GRID_VERSION,
   GRID_VERSION_KEY,
@@ -192,10 +193,30 @@ describe('grid-layout-migration (quick-260916-bwo)', () => {
     expect(r.layouts.sm[0]).toEqual({ i: 'a', x: 4, y: 2, w: 4, h: 2 });
     expect(r.migrated).toBe(true);
 
-    expect(migrateGridLayouts(null)).toEqual({ layouts: {}, migrated: false, newer: false });
-    expect(migrateGridLayouts(undefined)).toEqual({ layouts: {}, migrated: false, newer: false });
-    expect(migrateGridLayouts(42)).toEqual({ layouts: {}, migrated: false, newer: false });
-    expect(migrateGridLayouts([1, 2])).toEqual({ layouts: {}, migrated: false, newer: false });
+    expect(migrateGridLayouts(null)).toEqual({
+      layouts: {},
+      migrated: false,
+      newer: false,
+      canvas: null,
+    });
+    expect(migrateGridLayouts(undefined)).toEqual({
+      layouts: {},
+      migrated: false,
+      newer: false,
+      canvas: null,
+    });
+    expect(migrateGridLayouts(42)).toEqual({
+      layouts: {},
+      migrated: false,
+      newer: false,
+      canvas: null,
+    });
+    expect(migrateGridLayouts([1, 2])).toEqual({
+      layouts: {},
+      migrated: false,
+      newer: false,
+      canvas: null,
+    });
   });
 
   it('Test 8 (quick-260930): nur eine HOEHERE Marke meldet newer — v1/v2/v3 nicht', () => {
@@ -209,3 +230,58 @@ describe('grid-layout-migration (quick-260916-bwo)', () => {
     expect(future.layouts).toEqual(alt);
   });
 });
+
+describe('grid-layout-migration: Leinwand `__canvas` (Dashboard wie ein Bild mitskalieren)', () => {
+  const v3 = () => ({ lg: [{ i: 'a', x: 0, y: 0, w: 8, h: 4 }], [GRID_VERSION_KEY]: GRID_VERSION });
+
+  it('liest eine gueltige Leinwand und laesst sie aus `layouts` heraus', () => {
+    const r = migrateGridLayouts({ ...v3(), [GRID_CANVAS_KEY]: { w: 1600, h: 820 } });
+    expect(r.canvas).toEqual({ w: 1600, h: 820 });
+    expect(Object.keys(r.layouts)).toEqual(['lg']);
+    expect(r.migrated).toBe(false);
+  });
+
+  it('fehlende oder ungueltige Werte -> null', () => {
+    const bad: unknown[] = [
+      undefined,
+      null,
+      'x',
+      [1600, 800],
+      { w: 1600 },
+      { w: '1600', h: 800 },
+      { w: Number.NaN, h: 800 },
+      { w: Number.POSITIVE_INFINITY, h: 800 },
+      { w: 767, h: 800 },
+      { w: 1600, h: 199 },
+    ];
+    for (const value of bad) {
+      expect(migrateGridLayouts({ ...v3(), [GRID_CANVAS_KEY]: value }).canvas).toBeNull();
+    }
+    // Grenzwerte sind gueltig.
+    expect(migrateGridLayouts({ ...v3(), [GRID_CANVAS_KEY]: { w: 768, h: 200 } }).canvas).toEqual({
+      w: 768,
+      h: 200,
+    });
+  });
+
+  it('withGridVersion schreibt die Leinwand nur, wenn eine da ist', () => {
+    const { layouts } = migrateGridLayouts(v3());
+    expect(withGridVersion(layouts)).not.toHaveProperty(GRID_CANVAS_KEY);
+    expect(withGridVersion(layouts, null)).not.toHaveProperty(GRID_CANVAS_KEY);
+    expect(withGridVersion(layouts, { w: 1920, h: 950 })).toEqual({
+      ...layouts,
+      [GRID_VERSION_KEY]: GRID_VERSION,
+      [GRID_CANVAS_KEY]: { w: 1920, h: 950 },
+    });
+  });
+
+  it('Idempotenz mit Leinwand: Laden -> Speichern -> Laden liefert dasselbe, nichts skaliert', () => {
+    const once = migrateGridLayouts({ ...altLayouts(), [GRID_CANVAS_KEY]: { w: 1400, h: 700 } });
+    expect(once.migrated).toBe(true);
+    expect(once.canvas).toEqual({ w: 1400, h: 700 });
+    const twice = migrateGridLayouts(withGridVersion(once.layouts, once.canvas));
+    expect(twice.migrated).toBe(false);
+    expect(twice.layouts).toEqual(once.layouts);
+    expect(twice.canvas).toEqual(once.canvas);
+  });
+});